U.S. Pat. No. 5,510,209 is related to a solid polymer electrolyte-based oxygen battery. A polymer-based battery comprises metal anodes and an oxygen gas cathode. The oxygen is not stored in the battery but rather it accesses the battery from the environment. The solid-state battery is constructed by sandwiching a metal ion-conductive polymer electrolyte film between a metal anode (negative electrode) and a composite carbon electrode which serves as the cathode current collector on which the electro-active oxygen is reduced during discharge of the battery to generate electric current. The metal anodes include lithium, magnesium, sodium, calcium and zinc.
EP 1 261 048 81 is related to an electrode/separator laminate for galvanic cells and a process for its manufacture. According to the method disclosed in EP 1 261 048 81, a method is provided for producing an electrode/separator laminate for electrochemical elements which contains at least one lithium-intercalating electrode, which is composed of a PVdF-HFP-copolymer, wherein an electrochemically active material, which is insoluble in polymer, is finely dispersed.
The PVdF-HFP-copolymer is dissolved in a solvent and is mixed with electrochemically active materials. The pasty substance obtained in this way is extruded to form a sheet and is then laminated to a polyolefin separator which is coated with PVdF-HFP-copolymer. In each case, a PVdF-HFP-copolymer is used, having a proportion of HFP of less than 8% by weight. It appears to be very likely that in future battery systems, such as consumer or stationery application systems will be developed which are not based on intercalation, such as the established lithium/ion-technology. A promising battery technology which is in development is the Lithium/Air or Lithium/Oxygen system which makes use of a conversion process instead of an intercalation. Lithium/Air battery cells contain a metallic anode and an oxygen electrode and therefore realize a high specific energy cell level. A system with a metallic lithium-anode and an oxygen-electrode is described in afore-mentioned U.S. Pat. No. 5,510,209.
State of the art in electrode manufacturing processes are electrode binders such as polyvinylidene difluoride (PVdF) or polyvinylidene difluoridehexafluoropropylene (PVdF-HFP). Such polymer binders provide good adhesion between microscopic particles, however, establish electrical insulators. This in turn means that a composite electrode containing PVdF may show a decreased load capacity. Polymer electrode binders such as PVdF-HFP are well described in the literature as briefly has been discussed in EP 1 261 048 B1.
Electrode binders such as PVdF establish electrical insulators and even a few mass-% in the electrode composite may decrease the loading capacity of a battery cell considerably. If in battery cell designing, using electrode insulation material is disclaimed, one can only achieve very low loading by coating layers having a thickness between 100 nm and 1000 nm (1 μm). With such low loadings of active material, battery cells cannot be commercialized, because of the disadvantageous ratio between active and passive materials, not to name but a few such as collector foil, electrolyte and separators. For Lithium/Air technology under development, the level of carbon loading on the cathode (of oxygen electrodes) is limited to 1000 nm (1 μm). Above this thickness, the carbon atoms become unstable and show poor adhesion between the carbon particles and between carbon electrodes and metallic materials, such as the foil.
Taking into account theoretical and practical limitations of modern lithium-ion battery technology development, new approaches to increase battery performance are necessary. A promising approach is new battery electrochemical designs utilizing metal lithium as an electrode. Examples for this new electrochemical battery design are the system Lithium-Air/Lithium-Oxygen, Lithium-Sulphur, metal lithium-polymer. These technologies can achieve high specific energy. In the current state of the art, several problems have to be solved in each new battery electrochemical design. The problem of metal lithium protection is the most urgent one.
A review on current challenges for the system lithium/air was published in 2012 by Jake Christensen et al. in Journal of the electrochemical society, 159(2), R1R30 (2012). In this review, as the most promising way to protect lithium, are mentioned amongst others, lithium-conductive solid electrolytes. A huge list of lithium-conductive electrolytes has been narrowed to several variants due to the physical environment conditions of batteries: for example, a high lithium-ion conductivity is required, particularly higher than 10−4 Ohm−1cm−1 at room temperature (RT). Chemical and electrochemical stability, gas tightness, overall good mechanical properties, are required as well.
Among all, lithium-conductive solid electrolytes, only NASICON-based (Na Super-ionic Conductor) LATP (Li1+xAlxTi2-x(PO4)3) and LAGP ((Li1+xAlxGe2-x(PO4)3) (wherein x is equal to or greater than 0 and equal to or less than 2) are suitable. However, LATP is reported to be electrochemically not stable in contact with metallic lithium.
LAGP solid electrolytes are synthesized in two variants: ceramics and glass-ceramics. While NASI CON-based ceramics are the more explored of the materials, it allows very few ways to tailor material properties. Glass-ceramic is more promising due to initial built into gas tightness, being legacy of glass. Nevertheless, glass-ceramics electrolyte production is more complicated: non-uniform crystallization of glass results in a non-homogeneous distribution of such functional properties as ionic conductivity, thereby significantly decreasing performance of a battery. In addition, to uniform a glass crystallization of LAGP, a production of glass-ceramic with small crystal size is necessary for a creation of suitable solid electrolyte to be applied in lithium/air, lithium/sulphur and metallic lithium/polymer-batteries.